It depends on what you mean by "farm". If you are talking about a working farm with livestock and crops, then the value of the farm would go up because there would be more use for the land. However, if you are talking about a luxury home on a large piece of property, then the added value of having a tennis court may be negligible. The average rate for snow removal seems to range from $25-$50 per hour, depending on where you live. So if your farm is in an area that sees a lot of snow, then the cost of keeping the tennis court clear during the winter could be significant.
It depends on how much you value your farm and how much work you're willing to do to keep the tennis court clear in the winter. The average rate for snow removal is about $25 per hour, but it could be more or less depending on your location.